This invention relates generally to a method for creating a seal and/or an internal dam between an electronic and/or optical component mounted on a circuit board assembly and an enclosure housing, and to provide an integral sealing member for use in creating said seal and/or internal dam. More particularly, this invention is directed to the use of an integral sealing member, such as a gasket, exhibiting mechanical, physical, and thermal properties which enables the enclosure housing to be filled with a flowing encapsulation compound while inhibiting the blockage or leakage of encapsulating compound through access holes located in the housing prior to and during the curing of the encapsulation compound.
The manufacture of electronic packages or assemblies comprising a semiconductive element or an electronic component sealed to and encapsulated within a housing filled with a liquid encapsulation material, said material commonly consisting of a thermosetting organic polymer, which is subsequently cured to a solid, is well known in the art. U.S. Pat. Nos. 3,469,148 (Lund), U.S. Pat. No. 3,801,728 (Gallo, Jr. et al.), and U.S. Pat. No. 4,697,203 (Sakai et al) are illustrative of such devices. Lund is particularly instructive describing the basic features in the design of such devices. The patent concerns an integrated circuit assembly in which a semiconductive element is attached to a circuit board and wire bonded into the circuit on the board, with the resultant assembly being encapsulated in an organic plastic. Means are provided to isolate the enclosing plastic material from the filamentary wire bonds interconnecting the semiconductive element and the electrical surface. As illustrated there, a ceramic or plastic cover is bonded to the circuit board by means of an adhesive to isolate that portion of the circuit board enclosed within the cover such that the encapsulating plastic filling material cannot contact the surface portion of the circuit board within the cover. The bonding material is stated to be a low melting glass or an organic adhesive.
Gallo, Jr. et al. discloses the manufacture of microelectronic packages wherein polymide impregnated, woven glass fiber layers are utilized for bonding assembly members. Sakai et al. describe the fabrication of semiconductor devices wherein seals are provided via a thin film of a resilient material.
A problem occurs when an electronic or optical component such as an externally adjustable potentiometer, a light emitting diode (LED), or a liquid crystal display (LCD) is placed within a housing that will ultimately be filled with a liquid encapsulation material, such material frequently being referred to as xe2x80x9cpotting compoundxe2x80x9d. Prior to curing from a viscous liquid to a solid, the potting material goes through a low viscosity liquid phase where the material seeks to flow into undesired areas of the electronic or optical components, thereby either obstructing visual signals or seizing mechanical aspects of components requiring manual adjustments. Additionally, the encapsulant can flow out of the housing through unprotected holes therein, thereby necessitating reworking or scrapping of the final assembly. Finally, the encapsulant can flow into areas of the circuit board hazarding damage to electronic and/or electrochemical components.
The primary objective of the present invention is to provide a method for creating a seal and/or an internal dam between an electronic or optical component mounted on a circuit board assembly and an enclosure housing, wherein said electronic or optical component is externally adjustable, and/or has external leads, and/or performs a visual indicator function requiring an external access through the housing, said seal and/or dam being capable of preventing the flow of encapsulating material into undesired areas of the electronic or optical component and out through unprotected holes in the housing.
Another objective was to devise an integral sealing member for providing such a seal and/or internal dam which would act as a cushion to prevent damage from movement of the potting material during thermal cycling and from mechanical shock, vibration, and/or temperature extremes experienced in normal operation of the final assembly.
We have found that those and other objectives which will become apparent from the following description can be achieved through the use of an integral sealing member comprising a gel-type material exhibiting very low durometer/low shear stress such as to be basically a semi-solid, jelly-like substance, which material is capable of withstanding temperature extremes of xe2x88x9265xc2x0 C. to 155xc2x0 C., and which demonstrates adhesive tack for excellence in sealing.
General Description of the Invention
Prior to this invention, sealing of access holes in electronic assemblies of the type described hereinabove was conventionally performed utilizing one of the following materials or a combination of two or more thereof: dispensed sealants, cured foams, elastomeric gasket materials, or cardboard. Each material was subject to at least one serious drawback when employed to create a seal between electronic or optical components and the supporting housing. Such drawbacks included:
(1) The dispensing of a bead of material great enough to form a seal bridging a wide gap cannot be reliably and reproducibly accomplished during the assembly procedure, thereby leading to leakage of the potting material.
(2) Rigid, preformed gaskets fashioned from elastomeric compounds cannot fill the minor imperfections that may be present in the housing or the features which are necessarily molded in on the face of the electronic or optical component.
(3) It is customary to employ an adhesive to secure a preformed gasket to assure that it will not move during post assembly operations. The temperature at which the encapsulant (potting compound) is generally cured is not compatible with common adhesives, thereby resulting in leakage of the encapsulant.
(4) Preformed gaskets can become hard and brittle or they can shrink during the thermal cure of the potting compound which limits their ability to seal and cushion.
(5) A rigid spacer a gasket fashioned from elastomeric materials cannot reliably conform to the stack-up manufacturing tolerances presented in a formed or molded enclosure, in a circuit board, or in electronic/electromechanical components, as well as tolerances associated with the final assembly of the components.
We have found that the ideal sealant material for use in above-described package assemblies, and which is not subject to the above-described drawbacks, will exhibit the following mechanical physical, and thermal characteristics:
(a) it can withstand temperatures ranging from xe2x88x9265xc2x0 C. up to 155xc2x0 C.;
(b) it can be readily die cut punched, or otherwise shaped;
(c) it exhibits low durometer/low shear;
(d) it exhibits adhesive tack at ambient temperature to provide good sealing;
(e) it maintains sufficient fluid resistance during the curing procedure for the encapsulant to prevent leakage of the encapsulant; and
(f) is essentially a semi-solid, jelly-like substance, but which demonstrates sufficient structural integrity such that it does not creep or flow during the fabrication of the assembly.